1. Field of the Invention
The present invention relates to a surface profile measuring apparatus for measuring a surface profile of a sample, and more particularly to a surface profile measuring apparatus for illuminating a sample surface and measuring a surface profile of the sample based on reflection light from the sample surface.
2. Description of the Related Art
A surface glossiness is an important factor of the external appearance of an object, and is measured with respect to different kinds of products. The surface glossiness measuring method is standardized. As an example of the standardized method, FIG. 8 shows an arrangement of a surface profile measuring apparatus or a glossiness meter for measuring a surface glossiness at an incident angle of 60 degrees in accordance with ASTM D523. Referring to FIG. 8, a light flux from a light source 903, which has passed through an illumination aperture 902 having a rectangular opening in an aperture plate 901, is irradiated onto a sample surface 906 through an illumination lens 904, as a parallel light flux 905. Then, a specular reflection component 907 of light reflected on the sample surface 906 is converged on a light receiving lens 908, and the converged light is received on a light receiving device 911 through a light receiving aperture 910 having a rectangular opening in an aperture plate 909.
An optical axis 912 of an illumination system, and an optical axis 913 of a light receiving system are respectively tilted at 60° with respect to a normal 914 to the sample surface 906. The width (w) of the illumination aperture 902 in x-direction parallel to the plane direction of FIG. 8, and the height (h) of the illumination aperture 902 in y-direction orthogonal to the plane direction are respectively e.g. w=0.75 degrees and h=2.5 degrees in terms of angle of view. Similarly, the width (W) and the height (H) of the light receiving aperture 910 are respectively e.g. W=4.4 degrees and H=11.7 degrees in terms of angle of view. In the case where the sample surface 906 is a mirror surface, an image 902′ of the illumination aperture 902 is formed in the light receiving aperture 910, and the entirety of the reflection light from the sample surface 906 i.e. the specular reflection light component 907 is received on the light receiving device 911. However, as the glossiness of the sample surface 906 is lowered, the image 902′ is expanded. When the image 902′ is larger than the light receiving aperture 910, the light amount of light passing through the light receiving aperture 910, i.e. the light receiving amount of the light receiving device 911 is reduced. As described above, there is a relation between the glossiness and the light receiving amount. The surface profile measuring apparatus measures a glossiness value by utilizing the above relation i.e. correlating a light receiving amount to a glossiness value, and based on a difference in the light receiving amount. The measurement is performed based on the premise that the sample surface is flat. Accordingly, in the case where a curved surface of a sample is measured, a variation in measurement value may be increased, as compared with a case that a flat surface is measured. Japanese Unexamined Patent Publication No. Hei 8-128951 discloses an example of the above conventional art.
FIG. 9A is a diagram showing a condition that the cross section (S) of a projection light flux i.e. an incident light flux or the cross sectional size of a light flux with respect to a curved surface of a sample is relatively large in measuring the curved surface. Alternatively, the illumination area (P) of a portion where a projection light flux is projected may be used, in place of the cross section (S) of a projection light flux. In the case where the portion is illuminated with a circular light flux, the illumination area (P) corresponds to an illumination diameter. In this arrangement, outer light of the light flux 905 is reflected in a direction away from the light receiving aperture 910 i.e. the optical axis 913. Accordingly, it is difficult or impossible to accurately receive specular reflection light on the light receiving lens 908, thereby reducing the light receiving component. As a result, a variation in measurement value may be generated between a condition that a flat surface is measured, and a condition that a curved surface is measured. In other words, it is impossible to distinguish whether the decrease in the light receiving component is due to a variation in measurement value, or lowering of the glossiness. In view of this, there is proposed an arrangement, as shown in FIG. 9B, in which the cross section (S) of a projection light flux i.e. an incident light flux, or the illumination area (P) is reduced. In the arrangement shown in FIG. 9B, outer light i.e. reflection light of the light flux 905 is sufficiently close to the light receiving aperture 910. Accordingly, almost the entirety of the specular reflection light is trapped by the light receiving lens 908, and a variation in measurement value is significantly small between a condition that a flat surface is measured, and a condition that a curved surface is measured. In other words, reducing the cross section (S) of a projection light flux in measuring a curved surface enables to receive the light, even if the light receiving component may spread to some extent. This is advantageous in reducing a variation in measurement value.
In the case where the sample surface is a fine roughness surface i.e. an uneven surface, if the cross section (S) of a projection light flux is large, an averaging effect effectively works, and it is less likely that a variation in measurement value depending on the measuring position may be generated. Specifically, assuming that the reference numeral 921 in FIGS. 10A and 10B indicates a fine roughness surface, even if the position of projecting a projection light flux onto the fine roughness surface 921 i.e. the measuring position is displaced between a condition as shown in FIG. 10A and a condition as shown in FIG. 10B, the area of each white portion and the area of each black portion of the fine roughness surface 921 are identical in the range of the illumination area (P1), as far as the cross section (S) of a projection light flux is large. In FIGS. 10A and 10B, the area corresponding to two white portions and the area corresponding two black portions are identical. It should be noted that the fine roughness surface 921 conceptually represents an uneven surface. The white portions and the black portions in FIGS. 10A and 10B indicate small areas on the fine roughness surface 921, in other words, fine roughness or fine asperities. In other words, since the averaging effect works, and substantially identical reflection light is obtained between the condition as shown in FIG. 10A and the condition as shown in FIG. 10B, it is less likely that a variation in measurement value may be generated.
On the other hand, in the case where the cross section (S) of a projection light flux is reduced, a variation in measurement value may be increased. Specifically, in the case where the position of projecting a projection light flux onto the fine roughness surface 921 i.e. the measuring position is displaced between a condition as shown in FIG. 11A and a condition as shown in FIG. 11B, the area of a white portion in FIG. 11A and the area of a black portion in FIG. 11B on the fine roughness surface 921 may be different from each other in the range of the illumination area (P2), if the cross section (S) of a projection light flux is small. In other words, the averaging effect does not work, and a variation in measurement value may be increased, because reflection light is different between the condition as shown in FIG. 11A and the condition as shown in FIG. 11B.
In other words, in the case where the cross section i.e. the illumination diameter of a projection light flux is large, an error in measuring a curved surface may be increased, or measuring a small site or a small area may be difficult or impossible. On the other hand, in the case where the cross section of a projection light flux is small, a variation in measurement value may be generated even by a slight displacement in the measuring position, if the sample to be measured has an uneven surface such as a fine roughness surface. In view of this, it is necessary to use different kinds of measuring apparatuses with respect to each sample to be measured; or measure the sample a multiple number of times while changing the measuring position, and perform an averaging process with respect to the measurement values; or perform measurement while allowing a measurement error.